1. Field of the Invention
The present invention relates to an optical module with a flexible printed circuit board, in particular, the invention relates to an optical module with a specific connecting structure with a host board.
2. Related Prior Art
An optical module generally includes optical subassemblies (OSA) installing a laser diode (LD) or a photodiode (PD) and a circuit board installing electronic circuits to control the LD or the PD. The module is required with a small sized appearance and with a high transmission speed. The OSA provides a plurality of lead pins to guide electrical signals from/to the electronic circuits. In order to maintain the signal quality from/to the OSA, it is necessary to satisfy an impedance matching condition around the lead pins, namely, the lead pin itself and the wirings connected to the lead pins.
FIG. 8 shows a conventional wiring configuration between the circuit board 200 and the OSA 10 with lead pins 11. Because the lead pins 11 inevitably accompany with parasitic inductance, which causes an impedance mismatching condition, it is necessary to compensate this impedance mismatching with additional circuit components. When the transmission speed of the optical module enters Giga Hertz band and reaches 10 GHz, the impedance mismatching of the wiring pattern may be fatal to transmit a signal with good quality. One solution for such subject is to shorten the length of the lead pin and to use a flexible printed circuit (FPC) board between the lead pin 11 and the circuit board 200 on which impedance matched wiring patterns are implemented.
However, when the FPC board 100 connects the OSA 10 with the circuit board 200 and a portion of the FPC board 100 bent as the cupper foil of the wiring pattern is exposed, the copper foil is occasionally snapped. FIG. 8 shows an example to prevent the copper foil of the FPC board 100 from being snapped, where an additional adhesive 30 covers and supports the copper foil so as to bend the FPC board 100 at a portion R not covered by the adhesive 30.
A Japanese Patent Application published as JP-2005-050971A has disclosed a FPC board with additional cover film on a region between a bonding pad and an original cover film such that, when the FPC board is inserted with the lead pin of the electronic component into the bonding pad and bent at the region aside the bonding pad, the copper foil electrically connected to the bonding pad is not snapped. Another Japanese Patent Application published as JP-H05-061063A has disclosed a configuration to reinforce a portion where the FPC board is connected to the liquid-crystal panel by covering the contact portion with a resin curable with ultraviolet rays. Still another Japanese Patent Application published as JP-2004-193466A has disclosed a configuration of the FPC board in which, by covering a portion of the wiring pattern on the FPC board with silicone resin, stress likely to concentrate on the bent portion at an edge of the cover film may be relaxed.
In a conventional method where an additional film covers a boundary portion between the pad and the original cover film securing the pad, it is restricted in one direction for the FPC board to be bent, that is, the FPC board must be bent to a direction where the cover film comes inside. Moreover, the bending diameter of the FPC board must be large, which prolongs the wiring pattern on the FPC board and restricts the miniaturization of the optical module.
The method that uses the resin curable with the ultraviolet rays requires additional processes for irradiating the ultraviolet rays. Moreover, it is hard to bend a portion where the additional resin is cured, which makes a gap between the OSA and the substrate wide and prevent the miniaturization of the optical module.
The present invention is to provide a new arrangement of the FPC board connecting the OSA to the circuit board, which is configured to prevent the snapping of the wiring patterns formed on the FPC board by bending and to shorten the patterns to suppress the degradation of the high frequency signals transmitted on the wiring pattern. The invention also provides an optical module installing such new arrangement of the FPC board.